August 02, 2007

Isolated in Budapest under house arrest during WW 1 a Serbian astronomer and mathematician named Milutin Milankovitch wondered if there might be some relationship between the complex cycles of the Earth with the Sun as it moves and wobbles through space and the comings and goings of ice ages.

The difficulty was that these cycles were of widely different lengths
-20,000, 40,000, and 100,000 years- but each varying by up to a few
thousand years, which meant that Milankovitch had to compute the angle
and duration of incoming solar radiation at every latitude on earth, in
every season, for a million years, adjusted for three constantly
changing variables.

The result of his house arrest was the 1930 book Mathematical Climatology and the Astronomical Theory of Climate Change, which proved that he was right about the relationship between ice ages and planetary wobble.

We are all familiar with the notion that the Earth rotates on its axis
and revolves in orbit around the sun. There are at least five other
motions which complicate this simple picture: precession of the
equinoxes, precession of the Earth's orbit, axial tilt, eccentricity,
and obliquity of the Earth's orbit.

Only three variables, the precession of the equinoxes, axial tilt, and
eccentricity are thought to play a role in the Milankovitch cycle.

Precession of the equinoxes is the apparent change in stellar
background from one equinox to another -In popular terms, every 2000
years or so, the sun "moves" from one house of the zodiac to the
next--"This is the dawning of the age of Aquarius." The widely
accepted model for this precession states that it is caused by the
wobbling of the Earth's rotation around its axis, in a period that is
variously estimated as from 26,000 to 21,000 years.

Axial tilt is the inclination of the Earth's axis of rotation to the
plane of its orbit, an inclination which varies from approximately 21.5
degrees to 24.5 degrees from the vertical, and which is responsible for
seasons. As the Earth moves about its orbit, first one pole and then
the other is tilted toward the sun (at the equinoxes, the tilt is
parallel to the sun's axis). It has been discovered that axial tilt
varies with a period of about 41,000 years. We are currently
approximately in the middle of this period, with an axial tilt of 23.44
degrees.

Eccentricity refers to the shape of the Earth's orbit. Imagine the
orbit as showing a regular pulse from a shape more like a circle to a
shape more like a squashed circle, or ellipse, and you will have the
idea. The greater the eccentricity, the more like an ellipse. The
eccentricity of our orbit is primarily caused, according to our
calculations, by the gravity of Jupiter and Saturn. When the orbit is
more elliptical, the point of closest approach to the sun is closer
than when the orbit is more circular, and the point of farthest
distance from the sun is farther away.

According to the inverse square law, as the Earth approaches the sun
more closely, it receives more radiation, and as it moves farther away
it receives less.

Currently the eccentricity of the orbit is rather low, and at
perihelion when the Earth is closer to the Sun, the planet receives
only 6 or 7% more radiation than it does at aphelion.

In times of greater eccentricity, the variation may be as much as 20 or
30%. The major period of eccentricity is approximately 400,000 years
in length, with minor peaks occurring at intervals ranging from 95,000
to 136,000 years.

For the last million years, ice ages have occurred at intervals of
approximately 100,000 years, with lesser variations occurring at
intervals of 21,000 and 41,000 years. (Some scientist recently report
that landmasses which straddle the equator apparently experience cycles
of half the length of the 21,000-year cycle.)

For many years, it was unclear that there was a definable cycle in
climatological variations, but evidence from ice cores, cores from the
bottom of the ocean and ratios of carbon isotopes in organic strata
indicate that clear-cut cycles do exist, and many scientists take this
as confirmation of the Milankovitch cycle.

A question that has not been answered is the mechanism by means of
which the astronomical periods are reflected in the climate cycles.

The leading candidate for an explanation is the theory that since most
of the landmass is in the northern hemisphere, this landmass acts as an
accumulator for snowfall in periods of glaciation, and that ice ages
occur when warmer winters produce more water vapor in the atmosphere
(hence more snow) and cooler summers fail to melt the accumulations as
completely. (This theory would not account for climatic variations when
all the continents were bonded into the supercontinent we call
Gondwanaland).

Do the Milankovitch cycles mean that you might as well stop worrying
about global warming, since climate variation occurs in response to
astronomical forces and there is nothing we can do about it?

Not exactly. For one thing, no scientific consensus has been reached
on the validity of the Milankovitch cycles. While many scientists
consider the cycles factual, many others raise questions. The
"Transition Problem," for example, asks why, prior to a million years
ago, cycles occurred at 41,000-year intervals, but after a million
years ago, they occurred only at 100,000-year intervals.

It seems likely that climate is a complex system, and while
astronomical variations are likely to influence it, according to the
law of sensitive dependence on initial conditions it is very difficult
to isolate those influences and show a direct cause-and-effect
correlation.

For another thing, a significant body of scientists cautions that, even
if the Milankovitch cycles are accurate descriptions of the past,
current human activities may have an overwhelming affect on the
delicately balanced systems of the Earth's climate.

Comments

Milankovitch cycle relates to geographical condition of planet earth in solar system. The changes as per milankovitch cycle are natural and all powerful to which earth is adaptable.The present man made contributions are basically damaging the environment, making unsuitable for human existence.

Clearly we can survive a 240 foot rise in sea level easier than a wall of ice from New York to Seattle. Since we are at the tipping point for the beginning of a Milankovich 120k year ice age, why has no one suggested that we may have done well to dump the extra 100ppm of CO2 into the atmosphere to ward off the next ice age?

Ok, NEWS FLASH, the Earth "IS" warming, and the Earth HAS BEEN warming since the end of the last ice age. Chuck Baird is correct.

Can any of you answer this simple question.

There was a glacial ice sheet over 1 mile thick covering half of north America about 15,000 years ago and SOMETHING cause the Earth, at that time, to begin warming, what was it that caused the ice age to END?

The combustion engine has only been around for about 100 years and it wasn't used world wide until the middle of the 20th century.

Without doing ANYTHING, the combustion engine will be replaced by a better technology by the middle of this century. With recent advances in Graphene technology that promise to make batteries that can charge in minutes and discharge in hours, then electric cars will be much more economical for the average person.

Solution: Build a better technology and the world will LINE UP to buy it.

How is it possible that humans have volunteered to replace VCR's with DVD's and Bluerays without being FORCED to?
Answer: The technology was far superior for the same price.

You keep trying to make peoples lives SUCK.
I'm not taking a BUS, the solution is NOT to make life SUCK..